Patented Jan. 20, 1942 MULTICARRIE-R TRANSMISSION SYSTEM Thomas Samuel Skillman, Eindhoven, Nether lands, assignor, by mesne assignments, to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application October 6, 1939, Serial No. 298,331 In Great Britain October 10, 1938 1 Claim. 101. 179-15) bands isseparated by a filter'and modulated on a main carrier-wave in a second modulator. The frequency T of the main carrier wave for each speech to be transmitted is difierent and the main carriers T1, Tz-Tn are so selected that the frequency ranges occupied by the side bands to be transmitted are as close together as possible in the total frequency band to be transmitted.

According to the above patent the frequency S of the auxiliary carrier and the frequencies of the main carriers T1, TzTn are so selected that they are multiples of the same fundamental frequency a, which is determined by and is slightly wider than the width of the speech frequency band to be transmitted, and are in such relation to each other that the sumand difference-frequencies arising in the second modulation fall in two different frequency ranges. Thus, the sum frequencies T1+(S+a), T2+(S+a), Tn+(S+a), fall in a frequency range other than the difference frequencies T1(S+a) T2(S+a) ...Tn=(S+a), whereby it is supposed that after the first modulation in each channel the upper side band S+a is separated and applied to the second modulator. In this patent the two frequency ranges are separated by means of a filter connected between the output circuit of the second modulators of all the channels and the common transmission line so that either the sum frequencies T1+(S+a), Tz+(S+a) Tn+ (8+0) or the difference frequencies T1(S+a), Tz-(S-I-a) Tn(S+a) are transmitted to the receiver.

In such systems the output circuits of the second modulators are connected in parallel and this has the disadvantage that a large portion of the output of each of the modulators is lost in the other modulators and as a result only a small portion of the output energy reaches the transmission lines. Consequently, the efiiciency of the system is reduced.

Theobject of my invention is to overcome the above difiiculty and to increase the eiliciency of such systems and for this purpose I provide a simple band-pass filter in the output circuit of each of the second modulators.

In order that my invention may be clearly understood and readily carried into effect I shall describe the same in more detail with reference to the accompanying drawing in which the single figure is a schematic diagram of a multi-carrier transmission system embodying the invention.

In the drawing the speech currents received from the three subscribers I, 2 and 3 are fed through three low-pass filters 4, 5 and 6 respectively to three first modulators I, 8 and 9 respectively. The auxiliary carrier S, which is derived from a generator I0 if necessary through a frequency multiplier II, is modulated in modulators l, 8 and 9 by the speech currents. A side band S+a is separated by means of band-pass filters I2. I3 and I4 and is supplied to three second modulators I5, I6 and II, wherein main carrier waves T1, T2 and T3 are modulated successively by these side bands. The main carrier waves are also derived from the generator I0 and fed to the second modulators I5, I6 and through frequency-multipliers I8, I9 and 20. The output circuits of modulators I5, I6 and II are connected in parallel and are connected through a filter 2| to a transmission line 22.

Filter 2|, shown as a high-pass filter but which may be a low-pass filter, serves to eliminate the frequency range in which extend either the sum frequencies T1+(S+a), Tz+(S+a) and Tz+(S+a) o r t h e difference frequencies T1(S+a), Tz-(S+a).

In accordance with the invention I provide in the output circuits of the second modulators I5,

I6 and I1, band-pass filters 23, 24 and 25mspectively, which transmit one of the side bands arising in the corresponding modulators. The band-pass filter provided in the output circuit of one of the second modulators must produce only a comparatively low damping in the frequency range in which extend the side bands arising from the other two modulators. Thus substantially the entire output energy of each of the second modulators I5, I6 and I! will be fed to the transmission line 22.

The band-pass filter 23, 24 and 25 may be of very simple construction and need contain only a few elements so that they do not appreciably increase the costof the system. Thus the cost of the system according to the invention is even maintained considerably lower than that of the other known system, wherein the afore-mentioned relation between the frequencies of the auxiliary and main carriers does not exist and In a multichannel carrier wave system, a plu- 10 rality of channels each transmitting a conversation. a first modulator in each channel, means for supplying to each of said first modulators an auxiliary carrier having the same frequency in each of the channels, a first band-pass filter in each channel and separating one of the side bands produced in the first modulator of the channel by the modulation of the auxiliary carrier by the conversation, a second modulator in each channel, means for supplying to said second modulators the side band separated by said first band-pass filter, means for supplying to said second modulators main carriers having frequencies which are different for each of the channels and which are so selected that all the lower side bands and all the upper side bands produced in the second modulators lie in two separate frequency ranges, a common transmission line, and a second band-pass filter in each channel and connected between the second modulator of the channel and said transmission line, all oi. said second band-pass filters transmitting side bands produced in the corresponding second modulators and lying in the same one oi said separate frequency ranges.